Bjarke Rose

Laser-speckle angular-displacement sensor: theoretical and experimental study

A novel, to our knowledge, method for the measurement of angular displacement for arbitrarily shaped objects is presented in which the angular displacement is perpendicular to the optical axis. The method is based on Fourier-transforming the scattered field from a single laser beam that illuminates the target. The angular distribution of the light field at the target is linearly mapped on a linear image sensor placed in the Fourier plane. Measuring this displacement facilitates the determination of the angular displacement of the target. It is demonstrated both theoretically and experimentally that the angular-displacement sensor is insensitive to object shape and target distance if the linear image sensor is placed in the Fourier plane. A straightforward procedure for positioning the image sensor in the Fourier plane is presented. Any transverse or longitudinal movement of the target will give rise to partial speckle decorrelation, but it will not affect the angular measurement. Furthermore, any change in the illuminating wavelength will not affect the angular measurements. Theoretically and experimentally it is shown that the method has a resolution of 0.3 mdeg ( approximately 5 murad) for small angular displacements, and methods for further improvement in resolution is discussed. No special surface treatment is required for surfaces giving rise to fully developed speckle. The effect of partially developed speckle is considered both theoretically and experimentally.

Dynamic gate algorithm for multimode fiber Bragg grating sensor systems

We propose a novel dynamic gate algorithm (DGA) for precise and accurate peak detection. The algorithm uses a threshold-determined detection window and center of gravity algorithm with bias compensation. We analyze the wavelength fit resolution of the DGA for different values of the signal-to-noise ratio and different peak shapes. Our simulations and experiments demonstrate that the DGA method is fast and robust with better stability and accuracy than conventional algorithms. This makes it very attractive for future implementation in sensing systems, especially based on multimode fiber Bragg gratings.

A laser speckle sensor to measure the distribution of static torsion angles of twisted targets

A novel method for measuring the distribution of static torsion angles of twisted targets is presented. The method is based on Fourier transforming the scattered field in the direction perpendicular to the twist axis, while performing an imaging operation in the direction parallel to the axis. The Fourier transform serves to map the angular distribution of the scattered light field at the target into a linear displacement on a two-dimensional array image sensor placed in the Fourier plane. Measuring this displacement facilitates the determination of the angular displacement of the target. A cylindrical lens serves to image the closely spaced lateral positions of the target along the twist axis onto corresponding lines of the two dimensional image sensor. Thus, every single line of the image sensor measures the torsion angle of the corresponding surface position along the twist axis of the target. Experimentally, we measure the distribution of torsion angles in both uniform and non-uniform deformation zones. It is demonstrated both theoretically and experimentally that the measurements are insensitive to object shape and target distance if the image sensor is placed in the Fourier plane. A straightforward procedure to position the image sensor in the Fourier plane is presented. Furthermore, any transverse movement of the target will give rise to partial speckle decorrelation, but it will not affect the angular measurement. The method is insensitive to any wavelength change of the illuminating light source. No special surface treatment is required for surfaces giving rise to fully developed speckle.

Effects of target structure on the performance of laser time-of-flight velocimeter systems

The consequences of partially developed speckle and the effects giving rise to bias errors in velocity determination are discussed with respect to robustness of a classical laser time-of-flight velocimetry (LTV) system. It is demonstrated that surface regimes exist that define the degree of partially developed speckle. These regimes are explored both theoretically and experimentally; surface models are developed to predict the resulting cross covariance from which velocity estimations can be obtained. The surface models describe the behavior of the cross covariance caused by reflection structures and with disparate lateral-roughness scales. In particular, it is shown that it is possible to obtain a twin-Gaussian cross covariance as a result of the presence of partially developed speckle. All models described are compared with experimental observations of the cross covariance for differing surface regimes. The objects are solid targets having lateral spatial correlations in reflection amplitude, height, or both, generally giving rise to partially developed speckle. In almost all cases good agreement with the corresponding theoretical predictions are found. Decorrelation caused by velocity misalignment is shown to shift the peak of the cross covariance significantly, giving a velocity bias. A corresponding theoretical model is developed and verified experimentally. Cross-talk measurements have been performed and compared with a theory developed herein. Both measurements and theory indicate that only spot sizes comparable with or larger than their corresponding separation will lead to a measurable peak shift of the time lag for the maximum of the cross covariance. We conclude that LTV systems will provide accurate velocity estimates under a wide variety of practical conditions.

Performance of low-cost few-mode fiber Bragg grating sensor systems: polarization sensitivity and linearity of temperature and strain response.

We evaluate whether 850 nm fiber Bragg grating (FBG) sensor systems can use low-cost 1550 nm telecom fibers; in other words, how detrimental the influence of higher-order modes is to the polarization stability and linearity of the strain and temperature response. We do this by comparing polarization sensitivity of a few-mode 850 nm FBG sensor to a strictly single-mode 850 nm FBG sensor system using 850 nm single-mode fibers. We also compare the performance of the FBGs in strain and temperature tests. Our results show that the polarization stability and the linearity of the response degrade due to the presence of the higher-order modes. We demonstrate that, by using simple coiling of the 1550 nm fiber, one can regain the performance of the few-mode system and make it usable for high precision measurements.

Programmable spectroscopy enabled by DLP

Ibsen Photonics has since 2012 worked to deploy Texas Instruments DLP® technology to high efficiency, fused silica transmission grating based spectrometers and programmable light sources. The use of Digital Micromirror Devices (DMDs) in spectroscopy, allows for replacement of diode array detectors by single pixel detectors, and for the design of a new generation of programmable light sources, where you can control the relative power, exposure time and resolution independently for each wavelength in your spectrum. We present the special challenges presented by DMDs in relation to stray light and optical throughput, and we comment on the possibility for instrument manufacturers to generate new, dynamic measurement schemes and algorithms for increased speed, higher accuracy, and greater sample protection. We compare DMD based spectrometer designs with competing, diode array based designs, and provide suggestions for target applications of the technology.

Three-dimensional speckles: static and dynamic properties

A discussion of the static and dynamic 3D behavior of a speckle pattern is given. The discussion is based on general theoretical results, which are valid within the paraxial approximation for a general optical system using the concept of complex ABCD-matrices. Especially, we examine the 3D nature of speckles, which are formed in free-space, in Fourier transform geometry and in imaging systems. The dynamic evolution of these speckles is derived from the space/time-lagged intensity covariance that results from an in-plane translation of a diffuse scattering object, which is illuminated by a Gaussian shaped laser beam in a back- scattering configuration. In particular, the origin of speckle decorrelation will be investigated. Speckles are commonly analyzed in a plane where decorrelation can arise either from true speckle decorrelation or due to a frozen 3D speckle pattern being translated through the plane of observation, i.e. subjective speckle decorrelation. The presentation will be concluded with illustrative experimental results. The implications of the 3D dynamic properties for measurement systems based on speckle correlation will be considered.

Compact multichannel MEMS based spectrometer for FBG sensing

We propose a novel type of compact multichannel MEMS based spectrometer, where we replace the linear detector with a Digital Micromirror Device (DMD). The DMD is typically cheaper and has better pixel sampling than an InGaAs detector used in the 1550 nm range, which leads to cost reduction and better performance. Moreover, the DMD is a 2D array, which means that multichannel systems can be implemented without any additional optical components in the spectrometer. This makes the proposed interrogator highly cost-effective. The digital nature of the DMD also provides opportunities for advanced programmable spectroscopy.

An efficient and fast detection algorithm for multimode FBG sensing

We propose a novel dynamic gate algorithm (DGA) for fast and accurate peak detection. The algorithm uses threshold determined detection window and Center of gravity algorithm with bias compensation. We analyze the wavelength fit resolution of the DGA for different values of signal to noise ratio and different typical peak shapes. Our simulations and experiments demonstrate that the DGA method is fast and robust with higher stability and accuracy compared to conventional algorithms. This makes it very attractive for future implementation in sensing systems especially based on multimode fiber Bragg gratings.

Fiber-Optical Accelerometers Based on Polymer Optical Fiber Bragg Gratings

Fiber-optical accelerometers based on polymer optical fiber Bragg gratings (FBGs) are reported. We have written 3mm FBGs for 1550nm operation, characterized their temperature and strain response, and tested their performance in a prototype accelerometer.